Pestilence (6)
'pestilence' is the definition.
(I know that plague is a type of pestilence)
This is all the clue.
(Other definitions for plague that I've seen before include "Epidemic diseases with high mortality" , "Badger" , "Epidemic disease with a high death rate" , "Outbreak of a deadly disease, or infestation of pest like locusts" , "Lethal epidemic" .)
(Other definitions for pox that I've seen before include "illness" , "Disease that brings skin eruptions" , "Virus disease leaving pockmarks" .)